Understanding the slip deformation and crack initiation mechanism in a near-alpha titanium alloy during fatigue loading

Near-α titanium alloys, such as TIMETAL®834, have been developed for high temperature (up to 600°C) applications as high pressure compressor disks and blades of gas turbine engines. During service, high cycle fatigue (HCF), dominated by the crack initiation and early growth of microcracks, has proven to be the largest single cause of component failure. To obtain good ductility and high fatigue strength, TIMETAL®834 is typically produced with a bimodal microstructure consisting of equiaxed primary alpha (αp) grains located at the triple-point of the β grain boundaries and secondary alpha (αs) lamellae embedded in the β matrix. The role of different microstructural constituents in two-phase titanium alloys in crack initiation is still an area of great controversy. The aim of the present PhD project was to understand the interplay between early slip activities and fatigue crack initiation mechanisms in the context of critical microstructural features, e.g. αp volume fractions and macrozones, to predict preferential initiation sites under nominally elastic loading conditions. First, the exact nature of slip system activation and the associated shear strain contribution of different slip modes were investigated by EBSD-based grain orientation mapping in combination with high-resolution digital image correlation (HR-DIC) enabled relative displacement ratio (RDR) analysis and strain mapping. Basal <a> slip was identified to be the dominant slip mode due to the intrinsic elastic and plastic anisotropy of α-titanium. Regular appearance of two Burgers vectors that contribute to slip traces associated with the basal plane was revealed, for the first time, by a statistical analysis. Slip systems are regularly related to the 2nd highest possible Schmid factor demonstrating limitations of the highest Schmid factor's law in a polycrystalline material. Secondly, two types of cracking parallel to basal slip traces were observed by surface characterizations, i.e. transgranular cracks across αp grains and intergranular cracks at the grain boundary between αp grains. In addition, a distinct shift from transgranular to intergranular crack initiation was observed with increasing αp volume fractions and such intergranular cracks are related to slip initiating from (0001) twisted grain boundaries. Detailed 3D-EBSD results highlighted the essentially different facet forming mechanisms beneath the surface between two types of cracks. Transgranular crack facets developed in multi-steps at 6° away from the basal plane due to additional prismatic slip activation, while intergranular crack facets formed by an easy cleavage in one step along the basal plane. Statistical investigation demonstrated transgranular cracking always occurred in grains with a moderately high Schmid factor for basal slip, high resolved tensile stress along the c-axis and the Burgers vector being orientated strongly out-of-surface plane, while intergranular crack initiated at the (0001) twist grain boundary, a unique grain boundary configuration. These observations were used to develop models that can predict transgranular and intergranular crack initiation sites using a F parameter for considering the critical slip characteristics and G parameter identifying the (0001) twist grain boundary respectively. A further assessment showed that both parameters are effective for the prediction of crack initiation sites in the random texture regardless of the primary alpha volume fraction, while only G parameter is credible in the microtextured regions. Macrozones did enhance the transgranular crack formation by enabling easy basal </a> slip transfer reducing the requirement for out-of-plane shear, and higher local stress.